Technological Field
The present invention relates to an optical element and an optical connector suitably used for optical communication and the like.
Description of the Related art
Increases in size and in speed of information/signal processing have been in progress in various information/signal processing devices including a network device such as a router, a server, and a large computer. In these devices, signal transmission between a CPU and a memory in a circuit board (board), between wiring boards, between devices (racks), and the like has been conventionally performed by electrical wiring. However, in view of superiority in terms of transmission speed, transmission capacity, power consumption, radiation from a transmission path, interference of electromagnetic waves with a transmission path, and the like, so-called optical interconnection in which signals are transmitted by light using optical fibers or the like as a transmission path is beginning to be actually introduced in place of the above-described electrical wiring. In the optical interconnection, an optical connector is used to perform optical coupling between optical fibers. A typical optical connector has a lens that condenses light emitted from an end portion of one optical fiber to an end of the other optical fiber.
By the way, in recent years, the amount of optical communication information has been steadily increasing, and in addition, long-distance and high-speed transmission of information is desired. However, in the case of a conventionally used multimode fiber, optical fibers having core diameters of 50 μm and 62.5 μm are adopted, and an optical signal is transmitted in a plurality of modes. Therefore, a time lag is caused between arrival times of the signal and there will be an occurrence of mode dispersion. Therefore, a data loss occurs due to the mode dispersion, and thus the long-distance and high-speed transmission is considered unsuitable.
In contrast, a single-mode fiber is an extremely small-diameter optical fiber having a mode field diameter of about 9 μm, and has an advantage of suppressing attenuation as much as possible by setting propagation of an optical signal to one mode. Therefore, unlike the transmission method using many modes like the multimode fiber, the arrival time of the signal is single, and thus the single-mode fiber has no mode loss and is suitable for the long-distance and high-speed transmission. Therefore, opportunities to use the single-mode fiber have been increasing.
However, in using the single-mode fiber, the tolerance of core misalignment becomes narrow in optically coupling the optical fibers using an optical connector because the mode field diameter is as small as about 9 um. In addition, core misalignment may occur due to environmental temperature change. Detailed description will be given below.
A typical optical connector often couples multicore optical fiber bodies in which a plurality of cores is bundled together for the purpose of an increase in the amount of information. An optical connector used for such purpose generally includes a member for holding a multicore optical fiber body called ferrule, and an optical element disposed between a pair of ferrules and in which a plurality of lenses for effectively propagating light between a plurality of core ends held by the ferrules is formed.
However, even when the optical fibers are coupled using the optical connector with high precision, if a difference in thermal expansion occurs in each part due to the environmental temperature change, there is a risk of an increase in the loss due to misalignment of a coupled part, or the like. Here, the optical fiber is made of glass and the ferrule is often molded from a resin mixed with a glass fiber. The resin mixed with a glass fiber has a characteristic in which a linear expansion coefficient approximates that of glass. Therefore, a difference in thermal expansion can be said to less easily occur between the optical fiber and the ferrule. Meanwhile, the optical element is required to have a predetermined optical characteristic, and thus selection of material is a challenge. In the case where the lens is made of glass, for example, the linear expansion coefficient approximates those of the optical fiber and the ferrule, and thus the misalignment at the time of optical coupling less easily occur but the cost may increase. Further, since glass is generally difficult to mold, there is a challenge of how to provide a structure to position the glass-made optical element and the ferrule.
If the optical element is made of a resin containing a glass fiber by using the technique of JP 2016-133518 A, for example, a fitting hole for positioning can be formed with high precision, and the linear expansion difference between the optical element and the ferrule can be suppressed at the same time, thereby suppressing the efficiency loss when an environmental temperature changes.
However, it is technically difficult to match the refractive index and the temperature characteristics to a level that satisfies the optical characteristic in the material of the resin containing a glass fiber, and the resin is generally colored when mixed with the glass fiber, and the optical transmittance decreases due to the coloring. Therefore, it is difficult to use the resin containing a glass fiber for the optical element that is used for the optical connector.